The field of the invention is that of accelerometers and the invention relates more particularly to silicon micromachined accelerometers for regulating automotive safety air-bag systems.
Silicon micromachined accelerometers are known which comprise a member of silicon semiconducting material having a central mass mounted on an integral support by beams which extend between the mass and support to permit movement of the mass in response to acceleration force. Such known accelerometers have displayed less than desired sensitivity of response to acceleration force along a particular axis or have provided desired sensitivity only with a loss of some precision of response due to effects of off-axis acceleration forces or with significant loss of durability when subjected to dropping and the like during manufacture, shipping, storage, installation and use.
In one known accelerometer, a micromachined member of silicon semiconducting material has two beams extending from each of two opposite sides of a mass connected to an integral support to permit movement of the mass in response to acceleration force. Piezoresistive strain sensors accommodated in the beams provide an electrical output corresponding to acceleration of the device along an axis perpendicular to the plane of the mass and support. In that device, however, the mass tends to undergo substantial rotational movement due to off-axis acceleration forces so that the sensors in the different beams tend to indicate different degrees of mass movement. As a result, compensation in the device circuit must be relied upon to a significant extent to counteract the effects of that substantial rotational movement in order to provide acceptable accuracy in the output signal from the device. Most important, etching used in micromachining the silicon member leaves somewhat sharp corners at junctions of the beams with the mass and the support. When such beams are proportioned to provide the device with highly sensitive movement or acceleration response characteristics along one axis, the beams are very easily broken if they are subjected to significant off-axis acceleration forces such as will occur if the device is dropped during manufacture, shipping or installation or the like. That is, while the silicon material has substantial strength and has excellent spring characteristics for permitting highly responsive movement of the mass along the desired sensing axis, the silicon beam materials which are somewhat brittle are found to be easily broken by stress concentration at the noted sharp corners as a result of bending stresses due to the off-axis acceleration forces which are applied in the plane of the mass and support, transverse to the lengths of the two bars of beams, if the accelerometer is dropped.
In another known accelerometer as shown in U.S. Pat. No. 4,553,436, a folded leaf-spring portion of a micromachined silicon member extends between each of four sides of a central mass and a surrounding support but again the mass is subject to substantial rotational movement due to off-axis forces and the structure has poor resistance to damage during dropping. Another known accelerometer shown in U.S. Pat. No. 2,963,911 has a single straight beam extending from the center of each of four sides of a central mass to a surrounding support. In that arrangement the mass is subject to substantial rotational movement around an axis extending along the length of any beam or opposing pair of beams in response to off-axis acceleration forces. In another known accelerometer shown in U.S. Pat. No. 4,809,552, a single beam extending away from a mass divides into two legs where the beam connects to opposite sides of an integral support. That arrangement is subject to breakage of the legs if dropped due to off-axis acceleration forces in the direction along the length of the beam through the mass. In another accelerometer as shown in U.S. Pat. No. 4,825,335, more than ten beams extend between each of two opposite sides of a mass and a surrounding support and more than twenty beams extend between each of two other opposing sides of the mass and the support. Capacitive sensor means are arranged to detect movement of the mass. In that known structure, the multiplicity of beams tends to unduly restrict sensitivity of device response along the intended acceleration sensing axis of the device relative to the extent to which the beams oppose rotational movement of the mass. That is, the beams located near the central parts of the mass contribute little or nothing to oppose rotational movement of the mass.
Related subject matter is shown in commonly assigned copending application, Ser. No. 07/631,563 filed Dec. 21, 1990.